1. Field of the Invention
This invention relates to recording and reproducing a digital pattern as a sound track for a motion picture film, and the motion picture film with the digital pattern recorded thereon.
2. Background
In a conventional motion picture film such as shown in FIG. 16, the recording format has a large number of picture-recording areas (frames) 201 arranged in its longitudinal direction, that is along the direction of film travel (the film running direction). Audio signals related to the pictures recorded in the picture recording areas are recorded in the analog sound track areas 202 along side the picture-recording areas in the film running direction. This arrangement is prescribed in SMPTE standards. In the analog sound tracks, audio signals temporally coincident with the picture recorded in the picture recording areas (frames) are recorded at a position 20.5 frames ahead of the picture. This is done to account for the fact that the audio tracks are read by a playback assembly which is located between the projection assembly and the projector's take-up reel (20.5 frames ahead of the projected frame. During playback, the picture and associated audio are thus played back simultaneously.
In this conventional motion picture film, left and right channel audio signals are recorded with temporal correlation. As shown in FIG. 16, the audio signals are recorded in the analog sound track 202 at a position 20.5 frames ahead of the corresponding picture frame so that the audio signals can be temporally coincident with the picture recorded in the picture recording areas 201. However, a problem can occur if data are lost at the junction of hand-cut editing or due to scratches running vertically with respect to the film running direction, the playback capability of left and right channel audio signals is lost and therefore, sound interruption can occur. In other words, when a hand-cut edit is performed as a basic technique for film editing or repair, it may occur that audio signals are still left on the analog sound track 202 though a corresponding frame picture 201 has been already removed by editing, or that audio signals are cut out by editing though a corresponding frame picture still exists on the film.
Attempts have been made to provide areas for digital audio data in un-used areas of the standard format, in order to produce audio playback output during projection with sufficient ambience, while the current format is observed. The assignee of the present invention has already proposed a technique of providing two sound tracks in the film running direction on both sides of the picture recording areas for recording the motion pictures and digitally recording audio signals in these digital sound tracks for assuring sound reproduction with sufficient ambience. This technique is disclosed as a Japanese Laid-Open Patent Application Hei 5-40938 and a Japanese Laid-Open Patent Application Hei 5-109196 which was filed as a U.S. patent application Ser. No. 958,664 (abandoned) and its continuation application Ser. No. 090,705 and also another Japanese Patent Application Hei 4-320918 which was filed as a U.S. patent application Ser. No. 158,767, now U.S. Pat. No. 5,550,603. These applications are hereby incorporated by reference.
In such motion picture film, it is desirable to provide sound reproduction during projection with high sound quality and sufficient ambience, such that error correction or error concealment has to be performed on playback data reproduced from the digital sound tracks.
With conventional media, such as a digital equipment for a CD, cross-interleaved Reed-Solomon code is employed for error correction as protection against random errors or burst errors. The extent of such error correction capability required is set depending on the errors produced in the media.
The areas on the motion picture film for optical digital recording of audio signals are easily damaged by the film driving system. As the film is repeatedly reproduced a large number of times, a relatively long burst error can occur. Consequently, an interleaving length has to be set to be longer in order to eliminate sound interruption. However, when the interleaving length becomes longer, it causes phase offset between the picture on the film and its corresponding audio signal. This generates a limitation on a hand-cut editing which is a basic technique for film editing. Consequently, limitations are imposed on employing a longer interleaving length as a system.